The present invention relates to ultraviolet curable silicone coating compositions. More particularly, it relates to modified epoxy functional polydiorganosiloxane silicone fluids which are effectively cured by ultraviolet radiation in the presence of iodonium salt.
Silicone release coatings are used to render surfaces non-adherent to materials which would normally adhere thereto, and are widely used as coatings which release pressure-sensitive adhesives for labels, decorative laminates, transfer tapes, and the like. Such release coatings are advantageously formed by the cross-linking of epoxy functional polydiorganosiloxane silicone fluids in the presence of an ultraviolet (UV) cationic initiator. Suitable bis(aryl)iodonium salt cationic photocatalysts, for example bis(4-alkylphenyl)iodonium hexafluoroantimonate, have been disclosed in U.S. Pat. No. 4,279,717 to Eckberg et al., which is incorporated by reference herein. Another class of UV-sensitive catalyst is disclosed in U.S. Pat. No. 5,340,898 to Cavezzan et al., comprising tetrakis(perfluorophenyl) borate anions.
Despite extensive investigation into silicone release coating compositions and catalysts therefor, there remains a need for systems which provide faster, more efficient photocatalyzed curing of reactive epoxy functional silicones. U.S. Pat. No. 5,583,195 to Eckberg has reported synthesis and use of epoxy functional silicone polymers which have been partially or completely functionalized by a fluorescing, or a photosensitizing, or a simultaneously fluorescing and photosensitizing substituent bonded to the silicone polymer. These epoxy functionalized silicone derivatives are particularly useful for providing fluorescent dye marking compounds for determining the quality of the release coatings made therewith. In particular, Eckberg discloses reaction of an agent such as anthracene-9-carboxylate with an epoxy silicone to yield a functionalized epoxy silicone that possesses ultraviolet fluorescence activity and photosensitization activity. Such compounds are sensitizers for iodonium photocatalysts, promoting enhanced iodonium miscibility and enhanced photo cure response. Anthracenes however are expensive and therefore impractical for commercial use. Accordingly, there remains a continuing need in the art for commercially viable methods for the derivatization of epoxysilicones which will yield compounds of higher reactivity and excellent adhesion release coating properties.